(Technical Field)
The present disclosure relates to a power conversion systems. More particularly, the present disclosure relate to a power conversion system in which voltage is converted by a DC-DC converter to supply converted power to a power storage apparatus and an electrical load.
(Description of the Related Art)
The power conversion system has been employed in a vehicle system mounted on a vehicle such as an electric vehicle or a hybrid vehicle. For example, the electric vehicle or the hybrid vehicle is provided with a main battery used to supply power to main equipment that drives the vehicle and a sub battery used to supply low voltage power necessary for vehicle control or lighting, or to another electrical load. The sub battery is charged by being supplied power from the main battery. Since the output voltage of the main battery is higher than the output voltage of the sub battery, a DC-DC converter is used to step down the output voltage so as to supply power to the sub battery from the main battery. In this respect, to reduce energy loss of the DC-DC converter, Japanese Patent Number 379653 discloses a technique in which the output current of the DC-DC converter is compared with a predetermined value corresponding to maximum conversion efficiency of the DC-DC converter and the DC-DC converter stops outputting the power when the output current of the DC-DC converter is lower than or equal to the predetermined value.
In the above-described technique, after the output power of the DC-DC converter is stopped because the output current of the DC-DC converter is lower than or equal to the predetermined value and, when the output power is resumed, the output current of the DC-DC converter flows through the sub battery as a charge current and also flows through an electrical load. In this case, the DC-DC converter is designed such that the output current (i.e., charge current) is higher than or equal to the predetermined value even when the current flowing through the electrical load is zero. Therefore, while the electrical load has been driven by the battery, the output current of the DC-DC converter flows into the electrical load, whereby the output current of the DC-DC converter exceeds the predetermined value (current value corresponding to the maximum efficiency). Moreover, depending on state of the sub battery, charge current flowing into the sub battery becomes higher so that the output current of the DC-DC converter is likely to exceed the predetermined value significantly. In this respect, the inventor has confirmed that the conversion efficiency is decreased when the output current of the DC-DC converter is increased.